This invention relates to voice distribution and switching and, in particular, to combining voice distribution and switching with video distribution on a broadband coaxial cable network.
In U.S. patent application Ser. No. 635,751, assigned to the same assignee hereof, there is disclosed an improved system for providing voice and video distribution over a cable network. In the system of the '751 application, a plurality of telephone user or subscriber locations are provided and each subscriber location has an associated RF transmitting channel and an associated RF receiving channel. These channels carry voice and signalling information to and from, respectively, the subscriber locations.
A broadband cable network communicates with the subscriber locations and carries or couples with the associated RF transmitting and receiving channels. The cable network also communicates with a centralized switching and control means which is adapted to enable the RF transmitting channels to be selectively coupled or switched to any of the RF receiving channels. As a result, voice communication between the associated subscriber locations of the coupled channels can be carried out.
The particular switching means disclosed in the '751 application comprises a time division multiplex (TDM) switch and a frequency/time conversion means. The latter conversion means establishes transmitting and receiving TDM digital channels corresponding to the transmitting and receiving RF channels so as to permit switching between channels by the TDM switch. Also, in the system of the '751 application, the RF transmitting and receiving channels are analog channels organized into broadband RF channels of the cable network. Each user location, in turn, is assigned and couples with the broadband RF channel or channels containing its associated transmitting and receiving channels.
As can be appreciated, the conventional cable network used in the system of the '751 application includes many active and passive components, such as, for example, amplifiers, diplex filters, directional couplers, splitters and combiners. For such a cable network, standards have been established for its end-to-end characteristics. In particular, I.E.E.E. standard 802.7 requires that the network be designed such that it receives broadband RF signals in a standard RF channel (typically, a 6 Mhz channel) at a level of +54 dBmV and outputs these signals at a level in the range of +5 dBmV to +15 dBmV. This standard further requires that downstream signals (those in a receive channel) exhibit a signal-to-noise ratio of at least 43 dB, while upstream signals (those in a transmit channel) exhibit a signal-to-noise ratio of 41 dB.
When 900 (or 1000) standard voice channels (each, typically 4 Khz wide) are combined in one RF broadband channel (6 Mhz channel) in the '751 application system, the maximum signal power applied to each voice channel will be approximately 30 dB less than the total power which can be applied to the broadband channel. This is based upon an effective noise bandwidth for the 6 Mhz broadband channel of about 4 Mhz, since, as will be discussed, hereinbelow vestigial sidebands occupy part of this channel. Accordingly, the power of the noise in each 4 Khz voice channel will also be approximately 30 dB less than the noise carried in the broadband channel. Since both the signal and noise power each drop by 30 dB in each voice channel, the signal to noise ratio in each channel will remain the same as that for the broadband channel, i.e., 43 dB in the downstream direction and 41 dB in the upstream direction.
The aforesaid signal to noise ratios, however, are less than those typically recommended for conventional telephone communications. According to Transmission Systems for Communications, 4th edition, Bell Telephone Labs, 1969, p. 40, a typical telephone system operates with speech at -41.4 dBm, on average, with louder spurts at +10 dBm, or a ratio of 51.4 dB. Furthermore, according to CCITT recommendation G.222 for standard telephone systems, noise in a 4 Khz voice channel should be 2500 pW at a OdBm point. A standard 4 Khz telephone channel should thus exhibit a signal-to-noise ratio on the order of about 56 dB which, as, above-noted, is higher than that expected for the 4 Khz voice channels in the system of the '751 application.
It is, therefore, an object of the present invention to improve the system of the '751 application.
It is a further object of the present invention to modify the system of the '751 application so as to obtain higher signal-to-noise ratios for the voice channels of the system.